In recent years, with the object of enabling various portable electronic apparatuses such as a notebook computer and a cellular phone to be used for a long time without charging, attempts to use fuel cells as power sources of these portable apparatuses are made at various places. The fuel cell has a feature that electricity is generated by supplying only a fuel and air, and when only the fuel is resupplied, electricity can be continuously generated for a long time. Accordingly, if downsizing is realized, the fuel cell can be regarded as a system which is very profitable as a power source of a portable apparatus.
Particularly, in a direct methanol fuel cell (DMFC), methanol having high energy density is used as the fuel, and an electric current is directly derived from methanol by using a catalyst layer and a solid electrolyte membrane, and thus any reforming device is not required, downsizing is enabled, and the fuel can be handled easier than the hydrogen gas. Accordingly, the direct methanol fuel cell is promising as a power source of a small-sized portable apparatus.
As the supply systems of the fuel of the DMFC, known are a gas-supply-type DMFC in which a liquid fuel is vaporized and supplied to the inside of the fuel cell by a blower or the like, a liquid-supply-type DMFC in which a liquid fuel is directly supplied as it is to the inside of the fuel cell by a pump or the like, and an internal-vaporization-type DMFC in which a liquid fuel supplied to the fuel cell is vaporized inside the fuel cell and supplied to the fuel electrode.
As described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-106201, the internal-vaporization-type DMFC is provided with a fuel permeation layer for retaining the liquid fuel and a fuel vaporization layer for diffusing a vaporizable component of the liquid fuel retained in the fuel permeation layer, and in which the vaporized liquid fuel is supplied from the fuel vaporization layer to the fuel electrode. In the document described above, an aqueous solution of methanol obtained by mixing methanol and water at a molar ratio of 1:1 is used as the liquid fuel, both the methanol and water are supplied to the fuel electrode in a gaseous state. There has been a problem with such a fuel cell using an aqueous solution of methanol as the fuel that it is difficult to obtain a sufficient output characteristic because of a difference between methanol and water in the vaporization rate. Therefore, in order to improve the output characteristic of the fuel cell and further downsize the fuel cell, development of fuel cells using pure methanol as the fuel are now being advanced.
Further, as a small fuel cell mainly used in a mobile apparatus, a passive-type fuel cell in which an active transfer means such as a fuel pump is not used to supply a liquid fuel to a fuel electrode is also being developed. When a fuel cell is used in a mobile apparatus, it is difficult to employ a forced air-cooling means such as a fan, and thus heat generated by the bonding of hydrogen and oxygen must be discharged to the outside of the fuel cell by natural air cooling. If discharge of heat is insufficient and the temperature of the fuel cell becomes excessively high, electricity generation efficiency is lowered because of an increase in so-called crossover in which a liquid fuel such as methanol or the like passes from the fuel electrode to the air electrode, and further, the performance of the mobile apparatus main body is adversely affected or the portability of the mobile apparatus is impaired.